In the past, in a PABX accommodating multiple analog circuits and using multiple telephone numbers where operation is made by allocating a circuit and a telephone number to a certain extension group, the above operation has been executed by acquiring the circuit allocated by the control of the PABX when making a call from an extension number, and by allocating the receiving circuit for each telephone number by the control of the switchboard at the telephone office when receiving a call. Therefore, in this type of equipment, the ISDN primary rate interface circuit had to be handled as a group when accommodating the ISDN primary rate interface circuit, and a circuit could not be divided for a certain extension group and handled independently. For example, suppose there are three groups, A, B and C, as the communication channels and the calling parties, and suppose they jointly use 23 channels, if group A occupies many channels, there was a problem that B and C could not use those channels, thus the channel availability was restricted by the use by group A, a third party, which was inefficient.